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Another consideration is that the prop creates a pressure wave (it is a wing after all) that briefly blocks airflow flowing past it. Look at most props near the root where our cowl exits are; it is thick and rounded, it isn't moving much air back from the cowl exits!

Apparently having the cowl opening further away (cut back away from the prop) reduces this issue.

Air being a fluid will always flow via the least restrictive route (thus the need for baffles to force it through the cooling fins). And, it will flow from high to low pressure. If there is high pressure at your exit for any reason (and the aft end of bodies can be a bit chaotic in this regard), flow will be reduced. It might be interesting to experiment with a small 'flap' sticking outwards at the cowl exit to induce a low pressure behind it as a dignostic tool to determine if this makes a notable difference.

If there is constriction on the flow, it will slow or stop entirely. As an illustration, open a door (or window) on each side of your house (or other closed in building) on a windy day with the wind blowing towards one opening. Varying the opening of the exit side will clearly throttle the flow through the house.

As Barry noted as well, the air gets heated on the way through, so there is a greater volume to exit than enter!


Aerocanard (modified) SN:ACPB-0226 (Chapter 8)

Canardspeed.com (my build log and more; usually lags behind actual progress)
Flight simulator (X-plane) flight model master: X-Aerodynamics

(GMT+12)

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4 hours ago, Kent Ashton said:

Personally, I wouldn't bother with fireseal covering on fuel injection lines.  If there is a fuel leak, what's the ignition source?  The engine is not hot enough.  I suspect the fuel would evaporate as fast as it leaked out.

On an updraft cooled engine, if you put the distribution block and the FI lines on the top, they tend to get hot and cause vapor lock and very hard starting, as well as poor running when hot at low RPM/fuel flow. I've had customers have noticeably better engine running after installing insulation on the FI lines when they're on top of the engine. If you install the DB and FI lines on the bottom, as is recommended for updraft cooling, those issues don't exist (at least any more than they ever do on mechanical FI lycomings).

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9 hours ago, Marc Zeitlin said:

On an updraft cooled engine, if you put the distribution block and the FI lines on the top, they tend to get hot and cause vapor lock and very hard starting, as well as poor running when hot at low RPM/fuel flow.

Doooh!  I forgot about the heat issue.  I would think silver-covered firesleeve would work even better.  https://bmrswired.com/Firesleeve_tech_data.html

 


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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Lots of folks think a diffuser works on a canard.  I saw this one today being discussed on FB (pic 1).   The theory is that a diffuser will expand and slow the incoming airflow in an organized way and increased the pressure in the plenum under the engine.  I am skeptical.  The airflow into a NACA is swirling as it goes over the edges of the NACA inlet.  Then it is probably upset by the nose of a less-than-perfect inlet opening.  It is turbulent, swirling airflow..  As I recall, a surface intending to redirect an airflow and maintain its attachment to the surface needs to be no more than about  a 7 degree slope or the airflow will detach.  This is why a NACA floor is so shallow.  But that likely assumes a smooth airflow flowing over the slope.  The diffuser shown is far steeper than that and trying to diffuse turbulent stream of air.   Will it work?

It is fun to try these things but I'd recommend setting up a piccolo-tube/manometer arrangement above and below the cylinders and measuring the pressure difference before and after installation of the diffuser.  If there is no change, maybe you have wasted your time.  Comparing CHTs is problematic.  A difference in ambient temperature, or power setting, or lean setting or altitude makes it hard to compare using CHTs.   Diffusers sound high-tech but they are a complication and sometimes get in the way of working on things at the firewall.  I have tried a simple one (pic 2, alumimun) but danged if I could tell much difference.

I think what improves cooling is tight baffles and ramps that _force_ the airflow (air is heavy!) to go directly towards the cylinders, then having exits that let the air out.

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CozyIVEngLwr.jpg


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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1 hour ago, Kent Ashton said:

Lots of folks think a diffuser works on a canard.  I saw this one today being discussed on FB (pic 1).   The theory is that a diffuser will expand and slow the incoming airflow in an organized way and increased the pressure in the plenum under the engine.  I am skeptical.

I won't comment on the shown diffuser, but I will say that when I added a NACA diffuser, similar to that shown, to my cowl, I increased the pressure drop accross my oil cooler by anywhere from 5% to 13%, depending upon IAS.

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12 hours ago, Marc Zeitlin said:

I won't comment on the shown diffuser, but I will say that when I added a NACA diffuser, similar to that shown, to my cowl, I increased the pressure drop accross my oil cooler by anywhere from 5% to 13%, depending upon IAS.

Pictures please!


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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4 hours ago, Kent Ashton said:

Pictures please!

Here are a couple. This is a VERY simple diffuser - not 7 degree angles, and not very wide. It's got cutouts for hoses, throttle cable, and air filter, and yet, up to 13% more pressure at the oil cooler - never measured the pressure across the cylinders (as my CHT's were always fine).

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Pic grabbed from a FB page.  It looks like he has used 5.5" (66") or so of water differential to test his tanks  (arrows).  Way more than necessary.  It can separate the ribs from the top or bottom.  66" water differential is 2.38 PSI.  Puffer recommended about 21-24".  21" is .76 psi.  It is about the amount of pressure you can blow in with your breath.

tanktest.png


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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I don't think my friend overseas will mind me posting some pics.  He has a cooling problem so I asked him to send me some pics.  Pic 1 is the humongous metal scoop he added to help cooling.  Scary and shouldn't be necessary.  It might be something that goes through the prop someday.    Pic 2, Cowl mounted so the blue baffle material inside the upper cowl dam.  Plenum pressure will push the baffle inward and vent pressure.  Also a rather large gap at the side.  I neglected this once on my Cozy and the engine ran noticeably hotter.  Pic 3, gaps at the top of the forward right cylinder.  Pic 4, gaps in the top forward baffle and it appears the side baffle may not be tall enough to press against the dam.  Pic 5, gaps at the side and at the top baffle.  Also not a good seal (no baffle) where the metal meets the cowl at the exhaust pipes.  This is a hard place to seal.  So he is losing significant air through all these gaps.  Pic 6, an ugly swedge I would not trust.

He did not build this airplane.  The builder also used rather stiff baffle material which does not conform well.

Best to put a light in the cowls and night and look for things like this.

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-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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Reading this accident today:  Experienced pilots, long flight, low on fuel, voltage regulator dead,  battery dead or dying, tried to land at night with a fiance holding a flashlight at the end of the runway.  2 dead.   An aviation classic. 

http://www.kathrynsreport.com/2020/08/fuel-exhaustion-cessna-150h-n7152s.html


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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I had thought of something like this to increase climb cooling - but retract for cruise for "normal" NACA behavior.    I theorize that when NACA is hit by air in climb it actually turbulates the air and becomes a block of the opening and decreasing cooling efficiency in climb....
 
Hey - this is all speculation - but this picture does depict kinda what I was imagining (but having it articulate).
 
M.

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The study uses are Reynolds number of 1000. The Reynolds number on our use cases is in the millions. Not really applicable.

 

Varying the inlet has been shown to provide little or no gain for cooling, provided it is large enough to begin with. Varying the exit does help, but mainly to reduce drag when you are going fast and flow is enough with the exit reduced. The keys, as Kent has shown many times, are pressure differential (ie flow will actually move from inlet to outlet - you may be surprised how unwilling it is to do so in the somewhat chaotic flows at the aft end of these airplanes) and ensuring the air only goes where it needs to with proper working baffles.


Aerocanard (modified) SN:ACPB-0226 (Chapter 8)

Canardspeed.com (my build log and more; usually lags behind actual progress)
Flight simulator (X-plane) flight model master: X-Aerodynamics

(GMT+12)

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Tales of the Macabre, Aviation Edition:  A friend of mine took his Piper to a shop for avionics upgrades and then to a shop next door for an annual.  The mechanic shop said "you have low compression on one cylinder (55psi).  It will have to be replaced.  And you need major brake work."  My friend was a little upset.  Just a year earlier another shop that I trust had done a very thorough annual on his airplane (unfamiliar to them) and the compressions had been good.  I suppose the brakes were also good when they left that shop.  Last year's shop fixed some apparently deferred items and charged him about $6000 .

Now the current shop wants to install a new cylinder based on one compression test.  Mike Busch has written about this.  See "Risky Business" here  https://resources.savvyaviation.com/resources/mikes-articles/    Busch has said before that mere low compression is not cause to immediately replace a cylinder.   Check for metal in the oi filter, borescope the valves, and monitor it for some more hours;  sometimes the ring gaps have lined up or there is some other minor problem that can resolve itself.   But my friend let them do it.  Lycoming recommends break-in oil anytime a cylinder is replaced.  See Part II, ¶D here   www.lycon.com.icohttps://www.lycon.com/uploads/4/4/8/8/44889763/si_1014m_lycoming_recommended_oils.pdf

This shop told him "we don't use break-in oil, don't even keep it in stock".   Yeah, I know.  Not giving you a warm fuzzy feeling, right?    I knew a place nearby where he can buy the break-in oil so maybe he can convince the shop to change to the proper oil. 

We'll see what the final bill amounts to.  This makes me glad I do not have to deal with mechanics and especially mechanics that do not understand the difference between an "annual" and a "condition inspection".  I have replaced cylinders on my airplane.  At that time, I was not aware of all the cautions Busch writes about but I knew not to turn the prop. 


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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Sounds like a way to CHEAT the customer.   I got similar treatment on a car (that shop is now longer in business so I think they were desperate for money when I drove in).

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Friend of mine had a strikingly similar experience with a 172. One cylinder slightly low should have been no immediate cause for concern. They made him swap the cylinder, put in regular oil, and after several months the rings never seated. What baffles me is how many pilots seem to think a shops determination is gospel. He finally decided to rebuild the whole engine. He never asked my advice so I guess he's on his own. I operate an O470 and have seen an odd cylinder dip in compression just to come back to normal on recheck a few months later. I am a big fan of Mr. Busch. I do maintenance on condition and advocate a holistic approach in engine care; engine monitor data, borescope inspection, oil analysis, and of course the compression check. I list that last because it is least important. Read Mike Busch folks, he can save you a ton of money.

Edited by Tanker Pal
spell check

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we put a LOT of faith in Drs and Mechanics...  They are human beings - and NOT always looking after their customer as much as they are looking after the thickness of their wallet (or usually worse - the way to pay off the debt that they have dug themselves into). 

Guess we all agree - do your homework and get second opinions (or third for tie breakers 😉 )  We live in the world of a LOT of data - we have to sift thru it to find the information relevent to our situation!  (big fan of Mike Busch too!  sometimes he does come across as being a bit overconfident - but I guess that is what experience and being in the lime light causes..)

I know I wished I got second opinions (and listened to them) on some of MY bone head decisions!

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1 hour ago, mquinn6 said:

I know I wished I got second opinions (and listened to them) on some of MY bone head decisions!

Same here. I've made some doozies myself. "Good decisions come from experience, experience comes from making bad decisions." More and more, I try to learn from others bad decisions.

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Here's some of the cleanest baffle work you will see.  H/T Henry Herbert posting at  https://www.facebook.com/NorthwestCozy   More pics on his FB page.  I would avoid the dams where possible (pic 4,5).  It's a lot less work to let the baffles flop and they can be fitted for a good seal.  If the dams and the baffle material are not perfectly fitted, or a dam straddles a baffle (done that!) they can leak air.    Engines settle over time and move around under power.  Might affect the seal.  Still, nice job.  Very precise.

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-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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What is it about Nampa, Idaho?  For years I have seen ads for aircraft kit sellers, airplanes and projects at this airport.  Must be a hotbed of aviation.    Look at all the hangars!  When I die I want to go to Nampa  🙂

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-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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These are nice (pic) .  H/T Les Laidlaw.  Not sure they are worth $400 though.  .032 metal does not gain you anything and is harder to shape.  I use .025" 6061 which is a good alloy.  It is plenty stiff enough but can be bent fairly easily.   Baffles are a chore no matter how you make them and you might end up remaking some of them.  The process is very fiddly.

I got some offest printer plates free from a printer.  It is very thin metal and was useful for making test templates to cut out the final metal.  Or posterboard.  That'll work too.

baffles.png


-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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$400 for baffles being worthwhile depends where you are in your trade-offs between time and money, right? Looks like a nice kit.


Aerocanard (modified) SN:ACPB-0226 (Chapter 8)

Canardspeed.com (my build log and more; usually lags behind actual progress)
Flight simulator (X-plane) flight model master: X-Aerodynamics

(GMT+12)

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> Not sure they are worth $400 though.

That's about what Van's Aircraft charges for their RV baffle kit and I've never heard of anyone foregoing the kit in favor of sheet metal.   BTW, Van's RV-14 baffles cost twice that!

I would gladly spend $400 for Les Laidlaw's baffles except for the part about doing all the bending to fit.  And cutting out the holes for the alternator and starter.  And the inter-cylinder baffles, baffle seal material, and additional misc. items that always seem to be needed.  Guess I'll stick with what I have.  Yeah, no matter how you slice it, baffles suck.


Joe Dubner

Long-EZ, RV-8A

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